1. Field of the Invention
This invention relates to a method of thermal diffusion alloy plating for steel wire on a continuous basis, comprising the steps of plating steel wire with at least two different kinds of metal continuously in at least two layers of different metal and effecting thermal diffusion alloy plating on the steel wire plated in layers.
2. Prior Art
Conventionally, in the case where alloy plating is effected on steel wire with at least two different kinds of metal continuously, firstly steel wire is plated with different metals in layers in turn and then a thermal diffusion treatment is given to the plated steel wire for alloy plating.
The above method, however, causes irregularity in the quantity of metal plated, the plated alloy composition ratio (ratio of various metals composing the alloy plating in weight (%)) of plating composition gradient (degree of change of alloy composition ratio from the inner part to the outer part of alloy plating), etc. due to the changes in plating electric current efficiency (caused by changes in concentration of plating liquid, changes in plating voltage, wear of electrode poles, condition of an electrode plate, etc.), changes in diffusion voltage, changes in electric current, etc. Thus, it is very difficult to obtain the desired alloy plating with good precision and with uniformity in the lengthwise direction of the steel wire.
Especially in the case of steel wire to be used for reinforcing the rubber of tires for motor vehicles, conveyors, etc., strength and adhesion to rubber are essential to the steel wire and strict precision is required for alloy plating of this kind. More particularly, alloy plating for the steel wire is strictly limited in the quantity of metal plated and plated alloy composition ratio (ratio of Cu to Zn in weight, for example) for the aspect of adhesion to rubber and their value varies delicately according to the kind of rubber.
It is a fact that in order to improve adhesiveness, it is required to make changes between the inner part (central side) and at the outer part (outer circumferential side) and also to give an appropriate gradient to the composition ratio (composition gradient) from the inner to the outer side of the plating.
The quantity of metal plated, plated alloy composition ratio and the plating composition gradient affect rubber more or less and their effect on even one and the same rubber varies according to the condition under which the rubber is used. For example, in the case where rubber is used under a condition of high temperature, it is affected largely by the plated alloy composition ratio and in a wet condition, it is affected largely by the composition ratio. Therefore, it is necessary to take into consideration the combination of values of the quantity of metal plated, the plated alloy composition ratio and the plating composition inclination according to the kind, use, etc. of the rubber.
The precision required for alloy plating affects to a large extent the drawability in a drawing process which is carried out after alloy plating. Especially, the plated alloy composition ratio is related to breakage of wire in a drawing process.
A conventional method of alloy plating aiming at the desired quantity of metal plated, plated alloy composition ratio or plating composition gradient in good precision and in uniformity in the lengthwise direction of steel wire includes such steps that a thermal diffusion alloy plated steel wire is sampled for the purpose of analysis, and various analyses, including a fluoroescence X-ray analysis and analysis performed by the atomic absorption method, are carried out with the sample and on the basis of analytical results obtained, a plating electric current value in each plating tank for various metal plating is adjusted to obtain alloy plating having the desired quantity of metal plated and the desired plating alloy composition ratio.
Other methods available are, for example, the method of raising a temperature by Joules heat generation by electrifying steel wire directly, based on the idea that for obtaining the desired plating composition gradient uniformly, the diffusion temperature of steel wire is maintained at a fixed level by giving the prescribed heat value to the steel wire, and the method of indirect heating by making steel wire pass through a diffusion furnace. However, these methods have such a defect that even if the prescribed heat value is given for obtaining the desired plating composition gradient, the temperature of the steel wire itself does not always rise to the desired degree or such a defect that the composition gradient varies to a large extent due to a change in heating quantity, a change in the time of maintaining the raised temperature and a change in the thickness of the layer of each metal composing the alloy and the ratio of metals in weight.
For obtaining a steel wire of uniform alloy plating precision on a continuous basis by the above methods, it is suggested to carry out as frequent sampling and analyzing as possible, but since continuity is required for steel wire, this suggestion is not practicable.
Even if we try to manufacture brass plated steel wire of Cu conc. 65% by the above conventional methods, it is the utmost we can to limit the range within 62%-68% and therefore it is inevitable that we have irregularity of .+-.3% in the Cu concentration.
The relation between Cu concentration (Cu/Cu+Zn.times.100 (ratio in weight)) and adhesiveness to rubber and that between Cu concentration and drawability are as shown in FIG. 12(a). "Rubber adhering after adhesion test" in this figure represents the quantity of rubber adhered to the surface of steel wire which was left for two weeks at 80.degree. C..times.95% RH after it was plated with brass by thermal diffusion and subjected to a drawing process. The relation between the diffusion heating quantity obtained under the fixed Cu concentration and adhesiveness to rubber and that between the diffusion heating quantity and the drawability are as shown in FIG. 12(b). "Adhering of rubber after adhesion test" in this figure represents the quantity of rubber adhered to the surface of steel wire which was left for two weeks at 80.degree. C..times.95% RH after it was plated with brass at Cu conc. 65% and subjected to a drawing process.
As is obvious from FIGS. 12(a) and 12(b), adhesiveness to rubber and drawability have a reciprocal relation with each other. The fact that the irregularity of .+-.3% is inevitable in the conventional method is one of the reasons why steel wire of good alloy plating precision cannot be manufactured by the conventional method. It is therefore an important problem of the moment to lessen the irregularity in Cu concentration in the method of thermal diffusion alloy plating for steel wire by controlling the quantity of metal plated, plated alloy composition ratio and plating composition gradient.